1. Field of the Invention
The present invention relates to a Fiber-Reinforced Plastic (FRP) structure, and more particularly to a process for preparing a fiber reinforced plastic structure having at least one branching point such as, for example, a steering wheel core from a resin-impregnated strand of continuous long-staple fiber and to the product obtained from this the method.
2. Description of the Background
In the past the steering wheels of automobiles, for example, have been produced from polyurethane resin and polypropylene resin with the objective of reducing the weight thereof. However, since steering wheels formed solely of such resins do not offer the necessary strength, such steering wheels have instead been prepared by molding the resins with iron bars or iron pipes laid skeletally within the structure as its reinforcing core. Thus, in the production of steering wheels the weight of reinforcing material is normally about 40% of the total weight of steering wheel. These steering wheels do not meet the full objective of weight reduction, with a further disadvantage being that, depending on the intensity of vibrations exerted on the wheel, the outer resin structure of the wheel may separate from the reinforcing material.
As disclosed in the specification of British Patent Publication GB No. 2,004,835 A, a method has recently been described for the preparation of a shaped structure by winding a resin-impregnated fiber strand obtained by impregnating continuous long glass or carbon filters with polyester resin or epoxy resin on a suitably shaped frame or pattern. When a steering wheel is manufactured by forming a core of a resin-impregnated fiber strand of continuous long-staple fibers in accordance with this method, the steering wheel produced has the advantage that it is of a notably light weight because the resin-impregnated fiber strand is very much lighter than iron and the problem of separation between the outer resin and the reinforcing core, because of vibration, is eliminated because the resin-impregnated fiber strand is very tightly adhesive to the molded outer resin such as polyurethane.
When the reinforcing core for the steering wheel is prepared by the above-described method without any modification, a large void 4 tends to form where ring position 2, on which a resin-impregnated fiber strand 1 is wound in the circumferential direction of the wheel, and spoke portions 3, on which the same fiber strand 1 is wound in the radial direction of the wheel, join as illustrated in FIG. 7. When a bending stress or torsional force is exerted on the ring part 2, the applied stress is concentrated on the portion of the wheel where the void is at the juncture of the spokes and the ring portion with the result that cracks can occur between the branched resin-impregnated fiber strands. When the crack is left to grow gradually, a fracture may possibly result at the joint. A need therefore continues to exist for a method of manufacturing a fiber-reinforced plastic structure of improved structural and strength characteristics.